(a) Field of the Invention
This invention relates to a focus detecting device adapted to such optical instruments and medical instruments as photographic cameras, microscopes, hard endoscopes and retinal cameras.
(b) Description of the Prior Art
For this kind of focus detecting device, there has been conventionally adopted a light projecting detecting method to obtain a focus detecting device high in the response and precision. However, in the focus detecting device of the TTL system, as the projected light overlaps the observing visual field, an infrared light has been used. The visible light and infrared light are so greatly different from each other in the chromatic aberration that some correcting mechanism is required. This fact has become a great problem particularly in a microscope used by replacing objectives different in the chromatic aberration correcting amount. For example, in Japanese Patent Preliminary Publication No. 58-217909, it has been necessary to adjust the focus position of the infrared light by providing a lens system movable for a fixed distance in the optical axis direction. FIG. 1A shows a manner of forming an image with an objective X, the solid line indicating a visible light and the dotted line indicating an infrared light. FIG. 1B shows an image forming manner with an objective Y different from the above. As understood from these drawings, if the image by the visible light is made in the same position M, the image by the infrared light will be made in different positions Nx and Ny, because the chromatic aberration correcting amount is different with the objectives. As a result, the infrared light from a laser diode LD is incident as deviated on a detector D and is judged to be in an out-of-focus state. Therefore, it has been necessary to adjust the focus by moving a correcting lens C as shown in FIG. 1C.
As described above, in the prior art, the infrared light focusing position deviation has been corrected by moving a correcting lens but, as the focus position deviation is different with the objectives, it has been necessary to move the infrared light focusing correcting lens whenever the objectives are replaced.